1. Technical Field
The present invention relates to a technology for controlling behaviors or various driven elements, such as an organic light-emitting diode (hereinafter, referred to as ‘OLED’) element, a liquid crystal element, an electrophoresis element, an electrochromic element, an electron emission element, a resistor element, or the like.
2. Related Art
In general, various technologies for driving driven elements have been suggested. For example, in JP-A-2003-122301 (see FIG. 1), a structure is disclosed in which a plurality of unit circuits having OLED elements that serve as driven elements are two-dimensionally disposed. Each of the unit circuits includes a driving transistor that controls a current supplied to the OLED element according to a gate voltage, a reset transistor that causes the driving transistor to be diode-connected, and a light-emitting control transistor that switches whether the current is supplied to the OLED element or not. According to this structure, it is possible to compensate for an error (variation) of a threshold voltage of the driving transistor in each of the unit circuits.
Further, in general, various methods of driving driven elements have been suggested. For example, in JP-A-2003-223137 (see paragraph 0014 and FIG. 2), a structure is disclosed in which multi-gray-scale display is performed by controlling a pulse width of a driving signal (for example, current signal) supplied to an OLED element of each pixel. According to this structure, the pulse width of the driving signal is controlled for every pixel according to the comparison results between a data signal designating a gray-scale level of each pixel and a chopping-wave signal whose level temporally varies (hereinafter, referred to as ‘reference signal’).
Meanwhile it is preferable that the total number of transistors forming one unit circuit be small. The reason is as follows. If the total number of transistors is large, a structure of a unit circuit becomes complicated, which results in increasing the manufacturing cost. Further, in an electro-optical device that uses the unit circuit as a pixel, if the total number of transistors is large, an opening ratio may be lowered. However, according to the related art, there is a limit in that the total number of transistors in each unit circuit is reduced. For example, according to the structure disclosed in JP-A-2003-122301, in a period in which data is written in the unit circuit, a light-emitting control transistor needs to be provided so as to turn off the OLED element. According to an aspect of the invention, the invention is very effective in simplifying a structure of each unit circuit.
Further, according to a structure disclosed in JP-A-2003-223137 (see FIG. 2), one frame is divided into a scanning period and a light-emitting period, data signals are supplied to all pixels in the scanning period, and OLED elements of all pixels are simultaneously driven by supplying reference signals in the light-emitting period. As such, in a structure in which the scanning period and the light-emitting period are individually set in one frame, it is difficult to sufficiently ensure the time length of the light-emitting period. In addition, if the time length of the light-emitting period is insufficient, the luminance of each OLED element may be insufficient, and thus display becomes darker. Further, if the time length of the light-emitting period is short, a pulse width of a driving signal (in particular, pulse width corresponding to the low luminance) becomes short. However, in an electro-optical element, such as an OLED element or the like, a current may be concentrated in a short period (for example, a spike current may be supplied), and thus characteristic deterioration may occur.
Further, according to a structure disclosed in JP-A-2003-223137, a data signal is supplied to one electrode of a capacitor element. In this structure, it may be required to ensure a sufficient time for accurately setting a potential of the other electrode of the capacitor element when the data signal is supplied.